Tanning



Patented- UNITED STATES PATENT. OFFICE John Arthur Wilson,

Milwaukee, Wis, assignor to .Hall Laboratories Inc; Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. 'Application- June 17, 1937, Serial No. 148,757

11 Claims This invention relates to the manufacture of leather, and more particularly to the pretannage of hides'and skins with solutions of alkali-metal phosphates or their corresponding acids prepara-tory to their retannage with any of the other known tanning materials! The preferred-alkali-met'al phosphate which I employ as pretanning agent is .Graham's salt (Textbookof Inorganic Chemistry, edited by J. Newton Friend, vol. 6, part 2, page 177, by J. B. R. Prideaux, Philadelphia, 1934; Gmelin, Handbuch der Anorganischen Chemie, 'vol. 21, page 922, 8th edition, Berlin, 1928): or Graham's metaphosphate '(A Dictionary of Chemistry, by Henry Watts, vol. 4, page 5'78, New York, 1873; [A

Treatise on Chemistry, by Roscoe and soberlemmer, vol. 2, part 1, page 283, New York, 1923) Graham's metaphosphate or salt is believed to consist principally of sodium hexametaphosphate. The sodium hexametaphosphate is assumedto be a complex of the general formula Na2(Na4PcO1a), although some authorities believe that salts of the formula Na5(NaPeOia) and NadNazPeOm) may also be present.

Solutions which 'are similar to solutions of Grahams salt may be made by dissolving waterinsoluble Maddrell salts or Kurrol salts in acid solution as described in the 'copending application'- 1 of Casin ir .J. Munter, Serial No. 08,442, filed March- 12, 1936; These'solutions may be employed inplace of the solution of Grahams salt as a pretanningagent.

Grahams salt which consists principally of sodium hexametaphosphate, is a glassy- (noncrystalline)v sodium hexametaphosphate. Sodium. trimetaphosphate, on the other hand, is a crystalline material and although having an ap preciable tanning action, is considerably less'efv 'fective than Graham's salt. In fact, the trimeta- 40 phosphate has only approximately one-third the A tanning effect of Grahams salt. Another alkalimetal phosphate which exerts some tanning action but which is even less eifective than crystalline sodium trimetaphosphate, is sodium tripolyphosphate (NaaPaOm); Sodium tripolyphosphate is a definite chemical compound distinct from sodium metaphosphate or sodium pyrophosphate and is not a niixture of the two.

Sodium pyrophosphate has been found to have practically =notannin'g efiect on animal skin.

'The materials which I employ as pretanning agents in: accordance with time present invention have the general formula ,M2O.P2O5, in which M is hydrogen, ammonium and/or a alkali-metal, and in which the molarratio of mo to P205 is less than 211. In this formula, where M is-hydrogen, the material is an acid, and where M is an alkali-metal or ammonium, it is a phosphate. Sodium- 'metaphosphate (NaPOz), or sodium hexametaphosphate (NasPcOn) which is 5 known as Grahams salt, have a molar ratio of H940 to P205 of 1:1. Sodium tripolyphosphate (NasPaOm) has a molar-ratio of NazO to P205 of 5:3- Tetrasodium pyrophosphate (NSAPzO'I) has a molar ratio of NazO to P205 of 2:1, and 10 trisodium orthophosphate (NaaPOr) has'a molar ratio of NazO to P205 of 3:1. Neither the pyro phosphate nor the orthophosphate has an appreciable tanning action on animal skins and do not come within 'the scope ofthe present in- 16 vention.

The phosphates employedln my invention are complete tanning agents in themselves and, when used alone, produce pure white leathers of pile-- nomenal tensile strength, resistance to abrasion 20 and tearing resistance and of remarkable tight-r ness and fineness of grain and silkiness and length of fibers. I have found that these prop.- erties may be imparted, in large degree, to leathers of other tannages, which may be preferred 25 for certain uses, by pretanning the stock with phosphate before applying the other tannage.

As a typical example of the use of this pretannage for vegetable-tanned leather, I give the method which I developed for vegetable-tanned 3o goatskins. I put 1000 pounds of skins after ba'ting, before tanning, into a drum, which is' kept running during the entire period. of tenning. I add 20;pounds"of phosphorated cod oil and 40 pounds'of sulfonated cod oil in gallons 35' water at F. After 15 minutes, I add .40 pounds of Grahams salt in 20 'gallons water at 75 11; After 2 hours, I add sulfuric acid slowly until the'pH value of theliquor has been reduced to 2.5. Two hours after the last addition of acid, 0 I add 400 pounds of solid'quebracho extract in 250 gallons waterat 15 F. When the quebracho has completely penetrated the stock, as shown by examining a cutting, I remove the leather from the drum and treat and flnish'it according 45 do any one of the procedures employed foryegetable-tanned leather according to the type of. leather desired.

The phosphate pretannage greatly accelerates the tannage with quebracho and yields a leather {0 that is much stronger, more durable, of lighter and more uniform color and of finer feel than-is obtained when .quebracho is used without. the phosphate pretannaae'. Forchrome-t'anned leather, I proceed exactly as fo the vegetable-tanned leather up to and including the addition of Grahams salt. After 2 hours, I add sulfuric acid slowly until the pH value of the liquor has been reduced to 2.0. Four hours after the last addition of acid, I drain'ofi the liquor and wash the stock in running water for 1 hour to remove uncombined phosphate. I drain off the excess water and add basic chromium sulfate equivalent to 20 pounds of chromic oxide and -pounds of sodium chloride in 100 gallons water at 75 F. After 2 hours, I add sodium bicarbonate slowly over an hour to raise the pH value of the liquor to 3.25. Three hours after the last addition of bicarbonate, I remove the leather from the drum, pile it and allow it to stand for 24 hours. I then treat and finish it according to any one of the procedures employed for chrome-tanned leather according to the type of leather desired.

The resulting leather is more durable, of lighter color and finer feel than is obtained with chrome tanning alone.

For alum-tanned leather, I proceed exactly as for chrome-tanned leather up to and including the washing operation after the phosphate tannage. I drain off the excess water and add 125 pounds of aluminum sulfate and 60 pounds of sodium chloride in 125 gallons of water at 75 F. After 3 hours, I add sodium carbonate slowly over a period of 1 hour to raise the pH value to 3.95. Three hours after the last addition of carbonate, I remove the leather from the drum, pile it and allow it to stand for 24 hours. I then treat and finish it according to any one of the procedures employed for alum-tanned leather according to the type of leather desired.

Instead of employing either Grahams salt or other of the phosphate alone as the pretanning agent, I may employ mixtures of these phosphates, although, because of their superior tanning properties, the metaphosphates and particularly Grahams salt are the preferred pretanning agents.

I have given certain examples of the manner in which my invention may be carried out. Many variations in procedure may be made in order to make the product more suitable for special uses and the invention may be otherwise embodied or practiced within the scope of the following claims.

I claim:

1. A method of tanning animal skin, which comprises pretanning the skin with a'solution of a tanning agent. of the formula M20.P205, in

which, M is hydrogen, ammonium and/or an alkali-metal. and in which the 'molar ratio of M20 to P2O5is less than 2:1, and thereafter retanning the skin with a solution .of another tanning agent.

2. A method of tanning animal skin, which comprises pretanning the skin with a solution of a tanning agent of the formula M2O.P2Os. in

alkali-metal, and in which the molar ratio of M20 to P205 is less than 2:1, and thereafter retanning the skin with a solution of a chromium compound.

4. A method of tanning animal skin, which comprises pretanning the skin with a solution of a tanning, agent of the formula M20.P205, in-

which M is hydrogen, ammonium and/or an alkali-metal, and in which the molar ratio of M20 to P205 is less than 2: l, and thereafter retanning the skin with a solution of an aluminum compound.

5. A method of tanning animal skin, which comprises pretanning the skin with a solution of Grahams salt, and thereafter retanning the skin with a solution of another tanning agent.

6. A method of tanning animal skin, which comprises pretanning the skin with a solution of Grahams salt, and thereafter retamiing the skin with a solution of a vegetable tanning agent.

7. A method of tanning animal skin, which comprises pretanning the skin with a solution of Graham's salt, and thereafter retanning the skin with a solution of a chromium compound.

8. A method of tanning animal skin, which comprises pretanning the skin with a solution of Grahams salt, and thereafter retanning the skin with a solution of an aluminum compound. 1

9. A method of tanning animal skin, which comprises pretanning the skin with a solution of a tanning agent of the formula M20.P205, in which M is hydrogen, ammonium and/or an alkali-metal, and in which the molar ratio of M20 to P205 is less than 2:1, and thereafter retanning the skin with a solution of a vegetable tanning agent at a pH value below 3.5.

10. A method of tanning animal skin, which comprises pretanning the skin with a. solution of a'tanning agent of the formula M20.P205, in which M is hydrogen, ammonium and/or an alkali-metal, and in which the molar ratio of M20 to P205 is less than 2:1, the pretanning solution having a. pH value below 3.5, and thereafter retanning the skin with a solution of a vegetable tanning agent at a pH value below 3.5.

11. A method of tanning animal skin, which comprises pretanning the skin with a solution of Grahams salt at a pH value below 3.5, and thereafter retanning the skin with a solution of a vegetable tanning agent at a pH value below 3.5. 

